Synthesis of ribosomal RNA (rRNA) is catalyzed by DNA-dependent RNA polymerase I (Pol I). Pol I levels and properties have been compared in liver and hepatomas. Hepatomas contain a form of the enzyme which is distinct from that of normal liver. Two classes of hepatoma were observed. One class has 5 to 10 times more enzyme than normal liver while the second has levels similar to those of liver. Both classes of tumor synthesize rRNA at the same rate in culture. The enzyme in a high Pol I strain is structurally altered, as determined by thermal denaturation studies. Dexamethasone reversibly inhibits proliferation of lymphosarcoma P1798 cells in culture. A concomitant inhibition of initiation of transcription of rDNA is observed. Cell-free extracts of P1798 cells support specific initiation upon cloned mouse rDNA in vitro whereas extracts from hormone-treated cells do not. Data indicate that P1798 cells contain RNA polymerase I plus heatstable factors which are required for initiation of transcription of rDNA. One or more of these heat-stable factors is not expressed in hormone-treated cells.